Surface acoustic wave device and surface acoustic wave oscillator

ABSTRACT

A surface acoustic wave device, includes: an interdigital transducer serving as an electrode pattern to excite a Rayleigh surface acoustic wave, the interdigital transducer including a comb-tooth-shaped electrode having a plurality of electrode fingers; a piezoelectric substrate on which the interdigital transducer is formed, the piezoelectric substrate being made of a quartz substrate that is cut out at a cut angle represented by an Euler angle representation (φ, θ, Ψ) of (0°, 95°≦θ≦155°, 33°≦|Ψ|≦46°); electrode finger grooves formed between the electrode fingers of the comb-tooth-shaped electrode; and electrode finger bases being quartz portions sandwiched between the electrode finger grooves and having upper surfaces on which the electrode fingers are positioned. The surface acoustic wave device provides an excitation in an upper limit mode of a stop band of the surface acoustic wave.

BACKGROUND

1. Technical Field

The present invention relates to a surface acoustic wave device using anupper limit mode of a stop band of a Rayleigh surface acoustic wave anda surface acoustic wave oscillator including the surface acoustic wavedevice.

2. Related Art

An interdigital transducer (IDT) and a reflector that constitute asurface acoustic wave element include a plurality of conductive strips.The conductor strips have a periodic structure that provides a frequencyband, i.e., a stop band, to reflect a surface acoustic wave (SAW) of aspecific frequency domain by a high reflection coefficient,

A surface acoustic wave device made of an ST-cut quartz substrate with apredetermined in-plane rotated angle Ψ and enabling an excitation of anupper limit mode of a stop band of Rayleigh surface acoustic waves isknown for having an excellent frequency temperature characteristic.Further, a single type IDT only having two electrode fingers in awavelength of surface acoustic waves can be employed, as it is known,reducing difficulty in miniaturizing electrodes when a frequency isincreased and facilitating the increase of the frequency compared to acase of employing a reflection/inversion type IDT electrode (includingthree electrode fingers in a wavelength of surface acoustic waves) thatis required to excite an upper limit of a stop band in related art.Furthermore, it is also known that an excitation of an upper limit modeof a stop band has a smaller frequency variation when a thickness of anelectrode film is increased so as to reduce a resistance value of anIDT, i.e., a frequency variation caused by a variation of the thicknessof the electrode film, compared to an excitation of a lower limit mode(refer to JP-A-2006-148622 that is a first example of related art).

The first example of related art discloses that a SAW of the upper limitmode of the stop band becomes excitable when a propagation direction ofthe SAW is shifted from an X axis of quartz crystal.

In JP-A-11-214958 that is a second example of related art, it isdescribed that a stop band is formed by a periodic reflection of a SAWcaused by a periodic configuration in which a number of electrodefingers are aligned, and further a reflection/inversion type IDT is alsodisclosed in detail. Furthermore, according to the description,frequencies of a lower end (lower limit) and an upper end (upper limit)in the stop band are in a resonated state, generating standing waves.Positions of anti-nodes (or nodes) of respective standing waves of thelower limit mode and the upper limit mode are thus shifted from eachother.

Such a surface acoustic wave device disclosed in the first example ofrelated art can surely exhibit an excellent frequency temperaturecharacteristic, be suitable for increasing frequencies, and reduce avariation amount of the frequencies caused by a variation of the filmthickness. However, in the first example of related art, manufacturingerrors in mass production is not considered. For example, in amanufacturing process of a SAW device, when a resist pattern is formedand an electrode pattern is formed by wet etching, a difference in thethickness or the width of the resist pattern, and influence of sideetching proceeding etching from side surfaces of the electrode patternmay cause errors in a line width of electrode fingers constituting anIDT. When the electrode pattern is formed by dry etching, a variation inthe line width of the electrode fingers due to the side etching isreduced. However, a variation in the line width of the electrode fingerscaused by a variation of the thickness and the width of the resistpattern may occur similarly to the case of wet etching.

Further, in a surface acoustic wave device made of a quartz crystalsubstrate with an in-plane rotated angle as disclosed in the firstexample of related art, when a line metalization ratio η betweenindividual products varies due to manufacturing errors or the like, avariation amount of frequencies when a temperature is changed maylargely change. That is, a variation in a frequency temperaturecharacteristic becomes large. This becomes a major issue in reliabilityand quality of products in mass production in which a variation of linewidths occurs to no small extent.

SUMMARY

Advantages of the invention are to provide a surface acoustic wave (SAW)device that is suitable for mass production because a difference of afrequency variation caused by a temperature change between individualSAW devices when a line width of electrode fingers is varied, i.e., avariation of a frequency characteristic, is reduced, and to provide asurface acoustic wave oscillator including the SAW device.

The advantages of the invention described above can be realized asdescribed below.

A surface acoustic wave device includes an interdigital transducer atleast serving as an electrode pattern to excite a Rayleigh surfaceacoustic wave, the interdigital transducer including a comb-tooth-shapedelectrode having a plurality of electrode fingers; a piezoelectricsubstrate on which the interdigital transducer is formed, thepiezoelectric substrate being made of a quartz substrate that is cut outat a cut angle represented by an Euler angle representation (φ, θ, Ψ) of(0°, 95°≦θ≦155°, 33°≦|Ψ|≦46°); electrode finger grooves formed betweenthe electrode fingers of the comb-tooth-shaped electrode; and electrodefinger bases being quartz portions sandwiched between the electrodefinger grooves and having upper surfaces on which the electrode fingersare positioned. The surface acoustic wave device provides an excitationin an upper limit mode of a stop band of the surface acoustic wave.

This configuration can reduce divergence between peak temperatures of afrequency temperature characteristic of individual devices even whenerrors occur in a line width of the electrode fingers constituting theinterdigital transducer in a manufacturing process. Therefore, adifference of a frequency variation in a range of an operatingtemperature is reduced, allowing the surface acoustic wave device to besuitable for mass production That is, a variation of the frequencytemperature characteristic among individual surface acoustic wavedevices is reduced.

The surface acoustic wave device further may include a reflector formedat both sides of the interdigital transducer so as to sandwich theinterdigital transducer in a propagation direction of the surfaceacoustic wave on a surface of the quartz substrate. The reflectorincludes: conductor strips; conductor strip grooves formed between theconductor strips; and conductor strip bases being sandwiched between theconductor strip grooves and having upper surfaces on which the conductorstrips are formed.

This can improve reflection efficiency of the surface acoustic wave atthe reflector.

In the surface acoustic wave device, fr1<ft2<fr2 may be satisfied, wherean upper end frequency of a stop band of the interdigital transducer isft2, a lower end frequency of a stop band of the reflector is fr1, andan upper end frequency of the stop band of the reflector is fr2.

According to this, a reflection coefficient |Γ| of the reflectorincreases at the frequency ft2 of the upper end of the stop band of theinterdigital transducer, while the surface acoustic wave in the upperlimit mode of the stop band excited by the interdigital transducer isreflected towards the interdigital transducer with a high reflectioncoefficient at the reflector. Then, energy trapping of the surfaceacoustic wave in the upper limit mode of the stop band becomes stronger,thereby realizing a low loss resonator.

In the surface acoustic wave device, the conductor strip grooves of thereflector may be shallower than the electrode finger grooves of theinterdigital transducer in depth.

This enables the stop band of the reflector to shift towards a higherfrequency than the stop band of the interdigital transducer. Therefore,the relation of fr1<ft2<fr2 can be realized.

In the surface acoustic wave device, a relation of y and a value Hd/Hmay satisfyy=0.1825×(Hd/H)⁴−0.1753×(Hd/H)³+0.0726×(Hd/l)²−0.0058×(Hd/H)+0.0085,where a line metalization ratio η of the electrode fingers included inthe interdigital transducer is 0.8±y, and the value Hd/H is obtained bydividing a thickness Hd of the electrode finger base by a totalthickness H of the electrode finger and the electrode finger base.

In this case, based on the value Hd/H, a tolerance y of the linemetalization ratio that can favorably maintain the frequency temperaturecharacteristic can be obtained.

Further, in this case, the value Hd/H may be 0.167 or more. This cansuppress the variation of the frequency temperature characteristic in arange from 0 degrees Celsius to 80 degrees Celsius within 15 ppm.

In the surface acoustic wave device, the value Hd/H may be from 0.3inclusive to 0.833 inclusive. In this case, the frequency variation inthe range from 0 degrees Celsius to 80 degrees Celsius is made within 20ppm.

A surface acoustic wave oscillator includes the surface acoustic wavedevice according to the above, and an integrated circuit (IC) fordriving the interdigital transducer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIGS. 1A and 1B are diagrams showing a configuration of a SAW deviceaccording to an embodiment.

FIG. 2 is a diagram showing a cut angle of a quartz substrateconstituting the SAW device according to the embodiment.

FIGS. 3A and 3B are graphs showing differences of frequency variationswhen the line metalization ratios η of a SAW device in related art andthe SAW device of the embodiment are varied by ±0.1.

FIG. 4 is a graph showing differences of frequency variations when linemetalization ratio η of the SAW device of the embodiment is varied by±0.01.

FIG. 5 is a graph showing a frequency temperature characteristic of theSAW device having the line metalization ratio η at which the frequencyvariation is maximum when the line metalization ratio η is varied.

FIG. 6 is a graph showing a frequency temperature characteristic of theSAW device having the line metalization ratio η at which the frequencyvariation is minimum when the line metalization ratio η is varied.

FIG. 7 is a diagram showing transitions of the minimum and maximumvalues of the frequency variation when a depth of grooves is changed.

FIG. 8 is a graph showing a relation between the depth of the groovesand an error range of the line metalization ratio η when the grooveshave the depth in which the frequency variations among individual SAWdevices are within 20 ppm.

FIG. 9 is a graph in which an axis related to the line metalizationratio η in the graph shown in FIG. 8 is converted to an error of thewidth of the electrode fingers of an IDT in practice.

FIGS. 10A and 10B are diagrams showing a shift amount of a peaktemperature in a case where the line metalization ratio of the SAWdevice having the grooves is changed.

FIGS. 11A and 11B are diagrams showing a shift amount of a peaktemperature in a case where the line metalization ratio of the SAWdevice not having the grooves is changed.

FIG. 12 is a diagram showing SAW reflection characteristics of the IDTand the reflector.

FIGS. 13A and 13B are diagrams showing a configuration of a SAWoscillator according to another embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of a surface acoustic wave device and a surface acousticwave oscillator will now be described in detail referring to theattached drawings.

A surface acoustic wave (SAW) device 10 according to an embodiment is aresonance type SAW device basically including a piezoelectric substrate12, an interdigital transducer (IDT) 16, and reflectors 24, as shown inFIGS. 1A and 1B. As shown in FIG. 2, the piezoelectric substrate 12 ismade of a quartz substrate having crystal axes represented by an X axis(electrical axis), a Y axis (mechanical axis), and a Z axis (opticalaxis).

Here, Euler angle will be explained. A substrate represented as theEuler angle (0°, 0°, 0°) is a Z-cut substrate having a main surfaceperpendicular to the Z axis. In the Euler angle (φ, θ, Ψ) here, φrelates to a first rotation of the Z-cut substrate, and represents afirst rotation angle when the Z axis is a rotation axis and a directionto rotate from +X axis to +Y axis is a positive rotation angle. Further,θ of the Euler angle relates to a second rotation performed after thefirst rotation of the Z-cut substrate, and represents a second rotationangle when the X axis after the first rotation is the rotation axis anda direction to rotate from +Y axis after the first rotation to +Z axisis a positive rotation angle. A cut surface of a piezoelectric substrateis determined based on the first rotation angle φ and the secondrotation angle θ. Further, Ψ of the Euler angle relates to a thirdrotation performed after the second rotation of the Z-cut substrate, andrepresents a third rotation angle when the Z axis after the secondrotation is the rotation axis and a direction to rotate from +X axisafter the second rotation to +Y axis after the second rotation is apositive rotation angle. A propagation direction of the SAW isrepresented by the third rotation angle Ψ with respect to the X axisafter the second rotation.

In the embodiment, an in-plane rotation ST cut quartz substraterepresented by an Euler angle (0°, 95°≦θ≦155°, 33°≦|Ψ|≦46°) is employed.Such an in-plane rotation ST cut quartz substrate can constitute a SAWdevice with less frequency variations corresponding to temperaturevariation and an excellent frequency temperature characteristic.

The IDT 16 is provided with a pair of comb-tooth-shaped electrodes 18(18 a and 18 b) including a plurality of electrode fingers 22 (22 a and22 b) whose base portions are coupled by bus bars 20 (20 a and 20 b).The electrode finger 22 a constituting the comb-tooth-shaped electrode18 a and the electrode finger 22 b constituting the comb-tooth-shapedelectrode 18 b are alternately arranged at a predetermined interval.Here, the electrode fingers 22 are arranged in a direction orthogonal toan X′ axis that is the propagation direction of the SAW. The SAW excitedby the SAW device 10 constituted as above is a Rayleigh surface acousticwave. When the propagation direction of the surface acoustic wave isshifted from the X axis that is a crystal axis of quartz crystal asabove, the SAW in the upper limit mode of a stop band is successfullyexcited.

Further, the reflectors 24 are formed in pairs so as to sandwich the IDT16 in the propagation direction of the SAW. Specifically, a plurality ofconductor strips 26 which are formed parallel to the electrode fingers22 constituting the IDT 16 are coupled to each other at both ends.

Examples of an electrode material forming the IDT 16 and the reflectors24 constituted as above may include aluminum (Al), Al alloy, silver(Ag), tungsten (W), tantalum (Ta), copper (Cu), or an alloy includingone of these materials as a main constituent. When an alloy is used asthe electrode material, metal other than metal that is the mainconstituent should be included at 10% or less by weight.

The SAW device 10 according to the embodiment, which is basicallyconfigured as above, enables an excitation in the upper limit mode ofthe stop band since the propagation direction of the SAW is shifted fromthe X axis that is the crystal axis of quartz crystal as describedabove. Further, the SAW device 10 according to the embodiment haselectrode finger bases formed by digging a surface of the quartzsubstrate at a predetermined amount except for a portion of the surfaceto form the electrode pattern 14 so as to form grooves (grooves betweenelectrode fingers) 28 between the electrode fingers 22 of the IDT 16.This configuration can reduce variations of the frequency temperaturecharacteristic in a case where the line metalization ratio η of theelectrode fingers 22 constituting the IDT 16 varies. The linemetalization ratio η is obtained by dividing a line width d of theelectrode fingers 22 by a pitch p between the electrode fingers 22.Therefore, the line metalization ratio η is expressed by Formula 1.

$\begin{matrix}{\eta = \frac{d}{p}} & {{Formula}\mspace{14mu} 1}\end{matrix}$

Next, an example of a case where the grooves 28 are not formed betweenthe electrode fingers 22 and another example of a case where the grooves28 are formed between the electrode fingers 22 are described so as toshow differences of frequency variations depending on the configuration.

When a case of a SAW resonator having a resonant frequency of 322 MHz isexemplified, the thickness of an electrode film may be set at 0.6 μm asan example of a design value. On the other hand, in the case of the SAWdevice 10 (resonator) according to the embodiment, for example, half ofthe electrode film thickness, that is about 0.3 μm is compensated by adigging amount of the grooves 28 (Here, the grooves between electrodefingers 22 are defined as electrode finger grooves, while the groovesbetween the conductor strips are defined as conductor strip grooves.Hereinafter, the electrode finger grooves and the conductor stripgrooves may correctively be referred to as “grooves”.) and then bases ofquartz crystal (the electrode finger bases 30 and conductor strip bases32) are formed, so that the IDT 16 and the reflectors 24 with an actualelectrode film thickness of 0.3 μm are formed.

In this case, only the electrode finger grooves in the IDT 16 may beformed, but the conductor strip grooves in the reflectors 24 may not beformed. However, similarly to the embodiment, a case where not only theelectrode finger grooves in the IDT 16, but also the conductor stripgrooves in the reflectors 24 are formed is preferable because areflection coefficient of the SAW in the reflectors 24 can be increased.

FIGS. 3A and 3B show changes in the frequency temperaturecharacteristics between the case where the grooves 28 are not formed,and the case where the grooves 28 are formed between the electrodefingers of the IDT 16 or the like. FIG. 3A shows the case where thegrooves 28 are not formed between the electrode fingers, while FIG. 3Bshows the case where the grooves 28 are formed. In FIGS. 3A and 3B, avertical axis indicates variation amounts of the frequency while ahorizontal axis indicates temperatures.

In the both cases with the grooves 28 and without grooves 28respectively shown in FIGS. 3A and 3B, the line metalization ratio η isset at 0.8 as a target value and a case where the line metalizationratio η is varied by ±0.1 from the target value is exemplified. As it isfound in FIG. 3A, in the case without the grooves 28, when the linemetalization ratio η is 0.9, the frequency variation in a range of anoperating temperature is up to about 170 ppm. On the other hand, asshown in FIG. 3B, in the case where the grooves 28 are formed, even whenthe line metalization ratio η is 0.9, the frequency variation in therange of the operating temperature can be up to about 100 ppm. Further,the variation of the frequency temperature characteristic caused by thevariation of the line metalization ratio η is smaller in the case wherethe grooves 28 are formed. Accordingly, when the grooves 28 are formed,the maximum frequency variation can be reduced in addition to that thevariation of the frequency temperature characteristic caused by thevariation of the line metalization ratio η is reduced.

Here, in the examples shown in FIGS. 3A and 3B, the varied range of theline metalization ratio η is set at ±0.1 in order to facilitate theexplanation. However, the variation of the line metalization ratio η inan actual manufacturing process is about 0.01. In the SAW device 10(e.g. SAW resonator) manufactured with such accuracy, it is preferablethat the frequency variation be suppressed up to about ±50 ppm (100 ppmin an operating temperature range from −40 to 85 degrees Celsius) fromthe resonant frequency (e.g. 322 MHz) at a reference temperature (e.g.25 degrees Celsius) in a range between 0 and 80 degrees Celsius.Further, it is desirable that the frequency variation be suppressed towithin about ±10 ppm (20 ppm or less in width) in the range between 0 to80 degrees Celsius.

Under such relations and conditions, a method to obtain a relationbetween tolerances of the groove depth and the line metalization ratioη, and an optimum depth of the grooves includes the followings. That is,in a case where the piezoelectric substrate 12 made of quartz crystalhas the grooves 28 formed on a surface thereof in a predetermined depth,a target line metalization ratio η, the varied range of the linemetalization ratio η, and a frequency variation in a predeterminedtemperature range (e.g. between 0 and 80 degrees Celsius) when the linemetalization ratio η is ±0.01 are respectively obtained. Then, a maximumvalue and a minimum value of the frequency variation based on the variedrange of the line metalization ratio η are obtained. Thereafter, thegroove depth in a range of the frequency variation (e.g. within 15 ppm)allowing a difference between the maximum value and the minimum value isobtained.

Here, a graph shown in FIG. 4 shows a transition of the frequencyvariation when the depth of the grooves 28 is 0.3 μm, and the variedrange of the line metalization ratio η is ±0.01, while a total thicknessof the electrode finger base 30 and the electrode film of the IDT 16 anda total thickness of the conductor strip base 32 and the electrode filmof the reflectors 24 are 0.6 μm (a thickness of the electrode is 0.3 μm)respectively. In the temperature range that is from 0 to 80 degreesCelsius shown in FIG. 4, the maximum value of the frequency variation isobtained when the line metalization ratio η of the SAW resonator isvaried to the plus side, i.e., the line metalization ratio η=0.81.Specifically, in the temperature range described above, the frequencyvariation is 17 ppm (refer to FIG. 5). Further, the minimum value of thefrequency variation is obtained when the line metalization ratio η ofthe SAW resonator is at the target value, i.e., the line metalizationratio η=0.8. In this case, the frequency variation in the temperaturerange from 0 to 80 degrees Celsius here is 10 ppm (refer to FIG. 6).

Based on the result above, the total thickness of the electrode fingerbase 30 and the electrode film of the IDT 16, and the total thickness ofthe conductor strip base 32 and the electrode film of the reflectors 24are fixed at 0.6 μm respectively. Then, the digging amount of thegrooves 28 is calculated in a range from 0 to 0.4 μm (i.e. astandardized base thickness Hd/H obtained by dividing a base thicknessHd by the total thickness H of the electrode film and the base is from 0to 0.667). Values of the maximum variation (amount), the minimumvariation (amount), and the average amount are respectively plotted inthe graph in FIG. 7. In this case, the change of the depth of thegrooves 28 indicates a change of the thicknesses (heights) of theelectrode finger bases 30 and the conductor strip bases 32 and a ratio(proportion) of the thickness of the electrode film.

According to the graph in FIG. 7, when the line metalization ratio η isvaried by ±0.01, the depth of the grooves 28 needs to be 0.18 μm or morein order to obtain the frequency variation of 20 ppm or less in thetemperature range from 0 to 80 degrees Celsius. If this is representedby a relation with the thickness of the electrode film, when the totalthickness of the electrode finger base 30 and the electrode film, or thetotal thickness of the conductor strip base 32 and the electrode film is0.6 μm, the depth of the grooves 28 is more than or equal to 30% of eachthickness. That is, the standardized base thickness Hd/H should be 0.3or more. Further, in order to suppress the variation of the frequencytemperature characteristic within 15 ppm when the variation amount ofthe line metalization ratio η is ±0.01 (that is, a difference of themaximum value and the minimum value of the frequency variation), thedepth of the grooves 28 needs to be 0.1 μm or more (i.e., thestandardized base thickness Hd/H is 0.167 or more), and at least 0.5 μmor less (i.e., the standardized base thickness Hd/H is 0.833 or less).

In FIG. 7, in a range of the standardized base thickness Hd/H from 0.3to 0.667 inclusive, the data of which the frequency variation at thetemperature range from 0 to 80 degrees Celsius is 20 ppm or less isshown. That is, it is remarkably shown that the larger the standardizedbase thickness Hd/H becomes, the smaller the frequency variation causedby the temperature tends to be. Therefore, if the standardized basethickness Hd/H is more than 0.667 and at least 0.833 or less (0.5 μm orless), the frequency variation in the temperature range from 0 to 80degrees Celsius can be 15 ppm or less.

A tolerance (±y) of the line metalization ratio η to obtain thefrequency variation of 20 ppm or less in the temperature range from 0 to80 degrees Celsius is changed depending on the depth of the grooves 28.FIG. 8 shows a relation between the standardized base thickness Hd/H andthe tolerance (±y) of the line metalization ratio η to obtain thefrequency variation of 20 ppm or less in the temperature range from 0 to80 degrees Celsius is changed depending on the depth of the grooves 28.In FIG. 8, the tolerance (±y) of the line metalization ratio η to obtainthe frequency variation of 20 ppm or less in the temperature range from0 to 80 degrees Celsius can be found. Further, according to FIG. 8, asthe depth of the grooves 28 increases, the tolerance (±y) of the linemetalization ratio η becomes exponentially larger. That is, when theline metalization ratio η to obtain the frequency variation of 20 ppm orless in the temperature range from 0 to 80 degrees Celsius isrepresented by ±y, the followings should be satisfied as shown in FIG.8: η=0.8, andy=0.1825×(Hd/H)⁴−0.1753×(Hd/H)³+0.0726×(Hd/H)²−0.0058(Hd/H)+0.0085.

Further, the relation between the line metalization ratio η and thedepth of the grooves 28 shown in FIG. 8 is converted to a relationbetween an actual width of the electrode fingers 22 and the depth of thegrooves 28 as shown in FIG. 9. For example, according to the relationshown in FIG. 9, when an error of the width of the electrode fingers 22is ±0.1 μm, the depth of the grooves 28 needs to be 0.4 μm or more.

According to experiments, even when the frequency temperaturecharacteristic in the operating range varied due to an error of the linemetalization ratio η, a big difference was not found in a second-ordertemperature coefficient indicating the frequency temperaturecharacteristic. Resulting from this, it is found that a size of thefrequency variation varies due to a shift of a peak temperature in thegraph showing the frequency temperature characteristic. Therefore, byapproximating the peak temperature, that is, by reducing a shift amountof the peak temperature with respect to the reference temperature, aslope of the graph showing the frequency variation in a predeterminedtemperature range is also approximated. Further, such a configurationcan reduce the difference of the frequency variation between individualSAW devices, thereby improving total quality of the SAW devices to beproduced.

Here, graphs of FIGS. 10A, 10B, 11A, and 11B show whether a differencein a transition amount of the peak temperature occurs or not between theSAW device 10 including the piezoelectric substrate 12 in which thegrooves 28 are formed (FIGS. 10A and 10B) and a SAW device in which thegrooves are not formed (FIGS. 11A and 11B) when the line metalizationratio η is changed. Examples shown in FIGS. 10A, 10B, 11A, and 11B arealso examples of the SAW resonator having a resonant frequency of 322MHz and show a shift of the peak temperature when the line metalizationratio η is changed from 0.8 to 0775. As described above, according tothe graphs of FIGS. 10A, 10B, 11A, and 11B, it is found that even whenthe line metalization ratio η is changed, a large difference in thesecond-order coefficient showing the frequency temperaturecharacteristic is not found. In FIGS. 10A, 10B, 11A, and 11B, ameasuring temperature range of the frequency temperature characteristicis from 0 to 80 degrees Celsius.

In the examples described above, the peak temperature of the SAW devicein which the grooves 28 are formed is positioned at 35 degrees Celsiuswhen the line metalization ratio η is 0.8 (refer to FIG. 10A). Further,the peak temperature of the SAW device 10 in which the grooves 28 areformed shifts to a position of 64 degrees Celsius when the linemetalization ratio η is changed to 0.775 (refer to FIG. 10B).

On the other hand, the peak temperature of the SAW device in which thegrooves are not formed is positioned at 37 degrees Celsius when the linemetalization ratio η is 0.8 (refer to FIG. 11A). Further, the peaktemperature of the SAW device in which the grooves are not formed shiftsto a position of 130 degrees Celsius (out of the measuring temperaturerange) when the line metalization ratio η is changed to 0.775 (refer toFIG. 11B).

According to the above, it is found that the difference of the frequencyvariation between the individual SAW devices caused by errors of theline metalization ratio η in the manufacturing process of the SAW device10 can be reduced by forming the grooves 28 in the piezoelectricsubstrate 12 made of quartz crystal so as to have a depth correspondingto the film thickness of the excitation electrode 14. Further, suchreduction of the difference of the frequency variation between theindividual SAW devices is also realized by approximating the peaktemperatures between the individual SAW devices.

In the manufacturing process of the SAW device 10 having theconfiguration as above, metal which is a material forming the excitationelectrode 14 is deposited on a main surface of a wafer byvapor-depositing or sputtering first. Then, a shape of the IDT 16 andthe reflectors 24 are formed by wet etching or the like. Next, a surfaceof the piezoelectric substrate 12 other than a portion in which theexcitation electrode 14 is formed is dug out at a predetermined amountby dry etching so as to form the grooves 28. Here, the metal filmforming the excitation electrode 14 may be used as a mask when dryetching is performed.

After the process above is completed and the IDT 16 and the reflectors24 are formed on a main surface of the piezoelectric substrate 12, thewafer is divided to individual pieces by dicing or the like, so that theSAW device 10 is formed.

In the SAW device 10 having the configuration as the above according tothe embodiment, the shift amount of the peak temperature of thefrequency temperature characteristic of the SAW device 10 in an errorrange of the width of the electrode fingers 22 during the manufacturingprocess can be reduced by adjusting the depth of the grooves 28.Therefore, the difference of the frequency variation betweenmanufactured individual SAW devices is reduced, thereby enabling thefrequency variations of individual SAW devices to be within a tolerance.A manufacturing yield of the SAW device 10 is thus improved.

In the embodiment above, the total thickness H of the electrode film andthe base is set to be 0.6 μm. However, the invention is not limited tothis. Even when the total thickness H of the electrode film and the baseis other than 0.6 μm, if a Rayleigh SAW device made of a quartzsubstrate with the Euler angle (0°,95°≦θ≦155°,33°≦|Ψ|≦46°), and using anupper limit mode of a stop band can provide advantageous effects thatare similar to the embodiment above.

Further, as necessary, a protection film covering at least ones of theelectrode fingers and the conductor strips may be formed.

Further, in the embodiment above, it is described that the grooves 28are formed by digging out the whole surface of the piezoelectricsubstrate 12 except for the portion in which the excitation electrode 14is formed. However, in practice, a case where the grooves 28 are formedin a portion contributing an excitation, e.g. only a portion between theelectrode fingers 22 of the IDT 16, or the portion between the electrodefingers 22 of the IDT 16 and a portion between the conductor strips 26of the reflectors 24, should be regarded as a part of the invention.

Further, in order to efficiently trap energy of a surface acoustic waveexcited in an upper limit mode of a stop band, as shown in FIG. 12, anupper end frequency ft2 of the stop band of the IDT 16 may be set tobetween a lower end frequency fr1 of the stop band of the reflectors 24and an upper end frequency fr2 of the stop band of the reflectors 24.That is, a relation of fr1<ft2<fr2 can be satisfied. According to this,a reflection coefficient |Γ| of the reflectors 24 increases in the upperend frequency ft2 of the stop band of the IDT 16, while the SAW in theupper limit mode of the stop band excited by the IDT 16 is reflectedtowards the IDT 16 with a high reflection coefficient at the reflectors24. Then, the energy trapping of the SAW in the upper limit mode of thestop band becomes stronger, thereby realizing a low loss resonator.Here, when ft2, fr1, and fr2 are set so as to satisfy ft2<fr1 orfr2<ft2, the reflection coefficient |Γ| of the reflectors 24 becomessmall in the upper end frequency ft2 of the stop band of the IDT 16,thereby making it hard to realize strong energy confinement.

In order to satisfy fr1<ft2<fr2, the stop band of the reflectors 24needs to be shifted to a higher frequency than the stop band of the IDT16. This can also make an arrangement cycle of the conductor strips ofthe reflectors 24 smaller than an arrangement cycle of the electrodefingers of the IDT 16. Other than the above, this can be realized bymaking the film thickness of the conductor strips of the reflectors 24thinner than the film thickness of the electrode fingers of the IDT 16,or by making the depth of the grooves between the conductor strips ofthe reflectors 24 shallower than the depth of the grooves between theelectrode fingers of the IDT 16. Further, a combination of two or moreof these methods can be employed.

Further, in the embodiment above, only the SAW resonator is exemplifiedas an example of the SAW device 10, however, the SAW device of theinvention includes a SAW filter or the like.

Furthermore, the SAW device 10 described in the embodiment above is aresonator including the reflectors 24. However, the SAW device 10according to the embodiment includes an edge reflection type SAWresonator excluding reflectors.

A SAW oscillator according to the invention is, as shown in FIGS. 13Aand 13B, provided with the SAW device described above, an IC controllingan operation by applying a voltage to the IDT of the SAW device, and apackage accommodating them. FIG. 13A is a plan view when a lid isremoved and FIG. 13B is a view taken along a line A-A of FIG. 13A.

A SAW oscillator 100 according to the embodiment includes the SAW device10 and an IC 50 which are accommodated in a single package, a package56. The package 56 includes electrode patterns 54 a through 54 g formedon a bottom plate 56 a of the package 56, the comb-tooth-shapedelectrodes 18 a and 18 b of the SAW device 10, and pads 52 a through 52f of the IC 50 that are coupled to each other by a metal wire 60.Further, a cavity of the package 56 accommodating the SAW device 10 andthe IC is air-tightly sealed by a lid 58. In the configuration above,the IDT 16 (refer to FIG. 1), the IC 50, and an external mountingelectrode (not illustrated) formed on a bottom surface of the package 56can be electrically coupled.

The entire disclosure of Japanese Patent Application Nos: 2008-038951,filed Feb. 20, 2008 and 2008-287745, filed Nov. 10, 2008 are expresslyincorporated by reference herein.

1. A surface acoustic wave device, comprising: an interdigitaltransducer at least serving as an electrode pattern to excite a Rayleighsurface acoustic wave, the interdigital transducer including acomb-tooth-shaped electrode having a plurality of electrode fingers; apiezoelectric substrate on which the interdigital transducer is formed,the piezoelectric substrate being made of a quartz substrate that is cutout at a cut angle represented by an Euler angle representation (φ, θ,ψ) of (0°, 95°≦θ≦155°, 33°≦|ψ|≦46°); electrode finger grooves formedbetween the electrode fingers of the comb-tooth-shaped electrode;electrode finger bases being quartz portions sandwiched between theelectrode finger grooves and having upper surfaces on which theelectrode fingers are positioned, wherein the surface acoustic wavedevice provides an excitation in an upper limit mode of a stop band ofthe surface acoustic wave; a reflector formed at both sides of theinterdigital transducer so as to sandwich the interdigital transducer ina propagation direction of the surface acoustic wave on a surface of thequartz substrate, the reflector including conductor strips; conductorstrip grooves formed between the conductor strips; and conductor stripbases being sandwiched between the conductor strip grooves and havingupper surfaces on which the conductor strips are formed.
 2. The surfaceacoustic wave device according to claim 1, wherein fr1<f2<fr2 issatisfied, where an upper end frequency of a stop band of theinterdigital transducer is ft2, a lower end frequency of a stop band ofthe reflector is fr1, and an upper end frequency of the stop band of thereflector is fr2.
 3. The surface acoustic wave device according to claim2, wherein the conductor strip grooves of the reflector are shallowerthan the electrode finger grooves of the interdigital transducer indepth.
 4. A surface acoustic wave device, comprising: an interdigitaltransducer at least serving as an electrode pattern to excite a Rayleighsurface acoustic wave, the interdigital transducer including acomb-tooth-shaped electrode having a plurality of electrode fingers; apiezoelectric substrate on which the interdigital transducer is formed,the piezoelectric substrate being made of a quartz substrate that is cutout at a cut angle represented by an Euler angle representation (φ, θ,ψ) of (0°, 95°≦θ≦155°, 33°≦|ψ|≦46°); electrode finger grooves formedbetween the electrode fingers of the comb-tooth-shaped electrode; andelectrode finger bases being quartz portions sandwiched between theelectrode finger grooves and having upper surfaces on which theelectrode fingers are positioned, wherein the surface acoustic wavedevice provides an excitation in an upper limit mode of a stop band ofthe surface acoustic wave, wherein a relation of a value y and a valueHd/H satisfiesy=0.1825×(Hd/H)⁴−0.1753×(Hd/H)³+0.0726×(Hd/H)²−0.0058×(Hd/H)+0.0085,where a line metalization ratio η of the electrode fingers included inthe interdigital transducer is 0.8±y, and the value Hd/H is obtained bydividing a thickness Hd of one of the electrode finger bases by a totalthickness H of one of the electrode fingers and one of the electrodefinger bases.
 5. The surface acoustic wave device according to claim 4,wherein the value Hd/H is 0.167 or more.
 6. The surface acoustic wavedevice according to claim 5, wherein the value Hd/H is from 0.3inclusive to 0.833 inclusive.
 7. A surface acoustic wave oscillator,comprising: a surface acoustic wave device, comprising: an interdigitaltransducer at least serving as an electrode pattern to excite a Rayleighsurface acoustic wave, the interdigital transducer including acomb-tooth-shaped electrode having a plurality of electrode fingers; apiezoelectric substrate on which the interdigital transducer is formed,the piezoelectric substrate being made of a quartz substrate that is cutout at a cut angle represented by an Euler angle representation (φ, θ,ψ) of (0°, 95°≦θ≦155°, 33°≦|ψ|≦46°); electrode finger grooves formedbetween the electrode fingers of the comb-tooth-shaped electrode;electrode finger bases being quartz portions sandwiched between theelectrode finger grooves and having upper surfaces on which theelectrode fingers are positioned, wherein the surface acoustic wavedevice provides an excitation in an upper limit mode of a stop band ofthe surface acoustic wave; a reflector formed at both sides of theinterdigital transducer so as to sandwich the interdigital transducer ina propagation direction of the surface acoustic wave on a surface of thequartz substrate, the reflector including conductor strips; conductorstrip grooves formed between the conductor strips; and conductor stripbases being sandwiched between the conductor strip grooves and havingupper surfaces on which the conductor strips are formed; and anintegrated circuit for driving the interdigital transducer.